My landline provider offers a "call-control" feature. When someone calls, if the caller number has not been accepted before, a voice asks the caller to enter a randomly selected digit. Only after this has been correctly done is the call permitted to go through. Probably deflatable, but it has eliminated my robocalls. The only downside is that legitimate robocalls (eg: doctor's reminder) might be blocked if I have not whitelisted the number.

Simple solution, and I am surprised at how well it works.